The present invention relates to a pulse oximeter that is suitable to be attached to a living body during exercise for measuring a pulse rate and oxygen saturation (SpO2). The present invention also relates to signal processing applicable to even a case where a pulse cycle and a cycle of noise due to body motion induced by exercise become identical with or similar to each other.
Various methods have already been proposed as a method for separating a signal component and a noise component from two signals measured from a single medium almost simultaneously.
Under these methods, processing pertaining to a frequency domain and processing pertaining to a time domain are usually performed. In medical care, there have been known pulse photometers including an apparatus called a photoelectric sphygmograph for measuring a pulse wave and a pulse rate, an apparatus for measuring oxygen saturation SpO2 as measurement of concentration of a light-absorbing substance included in blood, an apparatus for measuring the concentration of abnormal hemoglobin, such as carbon monoxide hemoglobin, Met hemoglobin, an apparatus for measuring the concentration of injected pigment, and the like. Especially, the apparatus for measuring oxygen saturation SpO2 is called a pulse oximeter.
The principle in the pulse photometer includes determining the concentration of a target substance from a pulse wave data signal that is obtained by causing a tissue of a living body to reflect or allow transmission of light of a plurality of wavelengths, which exhibit different absorbing characteristic with respect to the target substance, and continually measuring intensities of transmitted or reflected light.
When noise is mixed into the pulse wave data, a correct concentration cannot be calculated, which will incur the risk of erroneous treatment. In the conventional pulse photometer, in order to reduce noise, there has hitherto been put forward a method for dividing a frequency band thereby determining a correlation between two signals contained in each of the divided frequency bands.
Japanese Patent No. 3270917 discloses a method for plotting two pulse wave signals, which are determined from transmitted light by irradiating a tissue of a living body with light of two different wavelengths, with the amplitude of one pulse wave signal being taken as a vertical axis and the amplitude of the other pulse wave signal being taken as a horizontal axis; determining a regression line of the signals; and determining oxygen saturation of arterial blood and concentration of a light-absorbing substance from the gradient of the regression line.
With this configuration, enhanced precision of measurement and a reduction in power consumption can be attained. However, large amounts of calculations are still required in order to determine a regression line and the gradient of the regression line through use of large amounts of sampling data pertaining to pulse wave signals of respective wavelengths.
Japanese Patent Publication No. 2003-135434A proposes a method that uses frequency analysis but includes determining a fundamental frequency of a pulse wave signal and further filtering the pulse wave signal by use of a filter employing a harmonic frequency of the fundamental frequency in order to enhance precision rather than extracting a pulse wave signal as in the conventional frequency analysis.
Japanese Patent Publication Nos. 2005-95581A and 2005-245574A propose methods for separating noise from a signal by use of a signal separation technique.
However, when noise due to the body motion of a subject, which is ten times as large as a pulse wave in terms of an amplitude ratio, is mixed, all of the above methods encounter difficulty in calculating a pulse rate and oxygen saturation of arterial blood, and further improvements have been desired.
Japanese Patent Publication No. 2007-83021A proposes, as an example of the improvements, a signal processing method in which there is lessened load on calculation processing for extracting a common signal component by processing two signals of the same type which are almost simultaneously measured from a single medium.
However, even in the case of the techniques described in the above publications, when the frequency of noise due to the body motion, such as exercise of a subject, becomes identical with or similar to the pulsation frequency and when the amplitude of noise is large, a pulse rate cannot be measured accurately.